Gambel oak influence on diameter increment of young-growth ponderosa pines was evaluated by intensively sampling three pine-oak stands on the Beaver Creek Watershed in north-central Arizona. Sampled stands had homogeneous climate, topography, soil parent material, vegetation structure and soil type. Increment cores were collected from five dominant pines on 34 randomly selected study plots. As revealed by dendrochronological techniques, radial growth of sampled pines had fluctuated around a relatively constant level during the last 50 years (1936-85). Differences in this level among stands were related to differences in competition, oak presence, pine age and site index. Pine diameter growth increased with increasing Gambel oak presence and with decreasing intraspecific competition.